A network element with an optical link can transmit multiple wavelengths on a single optical link in order to transmit more data on that optical link. In order to further increase capacity and extend reach of an optical link, multiple electrical signals are modulated into a single optical wavelength. For example, the optical signal can be a coherent optical signal where the multiple electrical signals are modulated into the optical wavelength. In this example, the electrical signals are modulated using polarization and quadrature to create a coherent optical signal on one wavelength from the electrical signals. In order to successfully demodulate the signals on the other end of the link, these signals need to arrive at the optical modulator at the same time. There also must be similar alignment on the receive physical interface (PHY). For example, if a pulse is transmitted on the four wires simultaneously, each of these pulses need to arrive within fractions of picoseconds in order for the pulses to be recovered on the receive PHY optimally. The amount of time difference in the pulse arrival between signals is known as skewing.
A pulse is delayed for a signal based on manufacturing variabilities that are inherent in the PHY, circuit board, connector interfaces and optical modules. A process known as de-skewing is performed to determine the amount of skew in the optical link and to configure the transmit and receive interfaces to remove this skew. The de-skewing process needs to be performed for each electrical interface that is manufactured for each network element. For example, the skew can be determined using a digital communications analyzer (DCA). The DCA measures the skew of the transmit PHYs and adjusts the skew to compensate for any difference between electrical signals. Properly de-skewed transmit signals then allows the receive PHY skews to be properly measured. This measurement can be performed by the PHY itself and requires no external hardware. With a de-skewed optical link, data transported in a coherent manner, arrive at the receive PHY within the allowed tolerances.
A problem with using the DCA to determine the transmit and receive PHYs skews is that this is an expensive process because the DCA itself is a very expensive and precise instrument. Introducing the DCA into the manufacture and calibration process of a network element with an optical interface increases the cost of manufacturing process for this device. Furthermore, the DCA is a precise instrument that is better suited in a lab environment rather than a manufacturing environment. This can mean that the DCA is not robust enough to be used long-term in a manufacturing environment leading to further cost due to DCA maintenance. Thus, using a DCA in such an environment increases the cost to produce and manufacturer network element with an optical interface.